The present invention relates generally to medical interventional devices adapted to be implanted in a patient's body, and more particularly to an improved implantable automatic defibrillator having a reduced energy threshold and which may include provision for other cardiac stimulus therapies including cardioversion and antitachycardia, bradycardia and rate-adaptive pacing for the implant patient.
According to the published literature, the first human implant of a completely implantable cardiac defibrillator took place in 1980. Since that time, considerable research has been conducted to provide improvements in such implantable medical devices, focusing on greater efficiency and smaller size. The principal constraints on size reduction have been and remain the batteries and the capacitors required to develop sufficient energy for defibrillating the patient's heart. To date, the large size of these components has dictated bulky and unwieldy stimulus generators, defying until recently any cosmetically acceptable pectoral implantation.
The minimum energy required for defibrillation of either atrium or ventricle constitutes the threshold of the device, which establishes a lower limit on its size. The nature of the delivery system consisting of the specific leads and electrodes and the electrode placement within or on the heart for application of the energy waveform, or shock, generated by the device contribute significantly to the threshold level. Given that the constraints on device size attributable to batteries and capacitors are a function of the technologies relating to those components, improvements in those areas have been left to the physicists and materials engineers. That effort is directed toward attaining long lifetime components with high energy storage capacity.
A parallel effort by cardiologists and biomedical engineers has been concentrated on improving the efficiency of the delivery system. The more efficient the delivery system in applying the shock energy developed by the stimulus generator to excite myocardial tissue for successful defibrillation with minimal energy loss--that is, the lower the threshold--the less energy required to be available from the batteries and capacitors, and consequently the smaller their size within the limits of the component technology. A lower threshold with attendant reduction of shock strength may provide additional benefits to the patient in the form of fewer physiologic side effects such as conduction disturbances, ventricular dysfunction and myocardial necrosis attributable to high energy defibrillation shocks.
The primary effort toward threshold reduction and consequent device size reduction has focused on lead/electrode systems, defibrillation waveforms, and energy vectors for shock delivery. From an original stimulus generator size/weight of approximately 240 grams (g), the state of the art in implantable defibrillators has progressed to a current size/weight of about 130 g.
Various efforts have been made in the past to improve the efficiency of energy delivery by means including use of the housing as a part of the lead/electrode system of the device.
In U.S. Pat. No. 5,133,353, an implantable cardiac stimulation lead system is proposed for pacing, cardioversion and defibrillation functions, in which the lead system includes a transvenous myocardial or pericardial lead having a plurality of electrodes, and the housing of the pulse generator is conductive and connected to the pulse generator circuitry so that it may serve as a discharge electrode. One side wall is a conductive mesh or has a conductive mesh fastened thereto. Such a configuration does not appear to provide any substantial benefits.
It is a principal object of the present invention to provide further reductions in energy threshold of implantable defibrillators, both atrial and ventricular, that would permit substantial size reduction in the stimulus generator, to lessen the onerous side effects on patient physiology and to afford greater cosmetic appeal for implantation in the pectoral region.
Another aim of the invention is to provide an implantable cardioverter/defibrillator in which the housing is used as a shocking electrode for defibrillation, in a manner significantly different from prior usage.
Yet another object is to provide a technique for delivering energy for defibrillating the heart in a way that enables selective concentration of the field to achieve low defibrillation thresholds.